<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style type="text/css">
        #test{
        /*    选择器1  */
            text-align: center;
        }
        p{
        /*    选择器2  */
            color: red;
        }
        .title{
        /*    选择器3  */
            font-weight: bolder;
        }

    </style>
</head>
<body>
<p id='test' class='title'>Hello world!</p>
</body>
</html>